1. Field of the Invention
The present invention relates generally to the field of utility and industrial boilers and furnaces and, in particular, to a new and useful integrated air foil and ammonia injection grid for use in a selective catalytic reduction (SCR) system which is used to reduce NOx emissions from such boilers and furnaces.
2. Description of the Related Art
NOx refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other species generated during combustion. Combustion of any fossil fuel generates some level of NOx due to high temperatures and the availability of oxygen and nitrogen from both the air and fuel. Once the fuel has been selected, NOx emissions may be controlled using low NOx combustion technology and postcombustion techniques. One such postcombustion technique is selective catalytic reduction (SCR).
SCR systems catalytically reduce flue gas NOx to N2 and H2O using ammonia (NH3) in a chemical reduction. This technology is especially useful if high NOx removal efficiencies (70% to over 90%/o) are required. FIG. 1 illustrates a typical utility boiler having an SCR system installed downstream (with respect to a direction of flue gas flow through the utility boiler) of the boiler flue gas outlet and upstream of the air heater.
As schematically indicated in FIG. 1, ammonia (NH3) is injected into and is mixed with the boiler flue gas via a grid 10. The NOx reduction reactions take place as the flue gas passes through a catalyst chamber (not shown) contained within the SCR. The NOx reactions with the NH3 can be represented as follows:4NO+4NH3+O2→4N2+6H2O  (1) 2NO2+4NH3+O2→3N2+6H2O  (2). 
For a general discussion of nitrogen oxides control in combustion processes, the reader is referred to Chapter 34 of Steam/its generation and use, 40th edition, Stultz and Kitto, Eds., Copyright© 1992 The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
The catalysts used in SCR systems are carefully engineered and expensive. Thus it is beneficial to be able to control the stoichiometry of the exhaust gas/ammonia/catalyst reaction. In such SCR systems, the ammonia is typically introduced into the flue gas stream using an injection grid system comprised of sparger tubes having a plurality of injection ports or nozzles. The unit-specific grid system is designed to provide an even distribution of ammonia throughout the flue gas. The particular grid system configuration is based upon the size of the flue conveying the flue gas stream, as well as the distance from the injection grid to the inlet of the catalyst bed of the SCR. Longer distances require fewer ammonia injectors since adequate mixing can occur prior to the mixture of flue gas and ammonia entering into the SCR catalyst bed.
Ammonia injection grids with zone control have been installed to distribute a prescribed amount of ammonia for NOx reducing SCR systems. Static mixers are also known, and have been proposed to reduce thermal and/or flue gas species gradients by adding turbulent mixing in SCR flue systems. Commercially available in several forms from companies such as Koch and Chemineer, these static mixers are used to improve the degree of mixing between the ammonia and flue gas prior to entry into the SCR.
FIG. 2 shows one known design for an ammonia injection grid 10. There may be multiple horizontal zones (N zones total) across a width of the flue which conveys the boiler exhaust flue gas 50. Ammonia injection grid 10 is comprised of multiple arrays 20 of sparger tubes 30, each having a plurality of nozzles 40. The nozzles 40 are arranged so as to inject the ammonia into and parallel with the flue gas 50 towards the catalyst located downstream (not shown). Groupings of the tubes 30 in a given array 20 are supplied from independently controlled supply headers 60. By varying the length of the tubes 30 and the position/orientation of the nozzles 40, the ammonia can be selectively injected into the what can be defined as an upper vertical zone A, a lower vertical zone B, or both, as required, and in any of the N−1, N, and N+1 horizontal zones as shown.
While the grid design 10 in FIG. 2 permits greater control over the dispersion of ammonia into the exhaust gas stream, it also results in blockage of a large area of the exhaust gas 50 flow path within the flue. The blockage in turn results in a large flue gas side pressure drop between the furnace and stack (not shown). This gas side pressure drop is undesirable because greater power consumption is needed for the fans to convey the flue gas through the installation, thereby adversely affecting the overall efficiency of the boiler system.
Air foils have also been used for mixing gas streams have been used in secondary air supply ducts and SCR system flues. FIG. 3 schematically illustrates this concept; for the sake of simplicity only one such air foil has been shown. In practice, the arrangement would comprise a plurality of whole foils 70 in the center portions of the flue and half foils at the walls of the flue. As shown, a first gas flow A approaches a rounded front end 80 of the air foil 70. A second gas flow B is provided into an interior portion 90 of the air foil 70. Air foil 70 is provided with a plurality of apertures 100 at a front portion thereof out through which the second gas flow B is conveyed, thereby mixing gas flow B with gas flow A downstream of the air foil 70. Air foils have also been used extensively for flow measurement and control.